This method is applicable in general to a liquid containing an unwanted gas in solution and it aims to reduce the concentration of the gas to a final concentration below its initial concentration. More particularly, it may often act to greatly reduce the concentration of air which is naturally dissolved in a mass of water that has been in contact with the atmosphere for a long time. Exchanges of air between the water and the atmosphere will by then have reached equilibrium and the initial concentration is thus the saturation concentration corresponding to atmospheric pressure. It is known that when a liquid capable of dissolving a gas is in contact with a gaseous medium containing the gas, gas exchange takes place continously through the interface between the liquid phase and the gas phase, and the exchange tends continously to approach a concentration of gas in the liquid known as the saturation concentration. It is also known that the saturation concentration is an increasing function of the pressure of the gas in the gaseous medium and that the mass of gas exchanged each second through the interface is proportional to the area thereof.
There may be various reasons for degassing: for example, to limit chemical reactions in which the gas might take part; or to avoid excessively facilitating the formation of pockets of gas in the liquid during subsequent localized reductions of pressure, eg. at the blades of a screw.
A known method of degassing is described in SOVIET INVENTION ILLUSTRATED, Derwent Publications Ltd., Week D39, 4 Nov. 1981, London (GB); and in Soviet patent specification 793 600 (Shleifer A. A., 7 Jan. 1981). This method comprises a mass exchange operation at reduced pressure during which the liquid to be degassed is placed in a free surface separation enclosure. The gaseous medium above the liquid is aspirated to reduce the pressure of the unwanted gas to a degassing value below its saturation pressure which corresponds to the desired final concentration, and this degassing pressure is maintained for the time required to obtain the desired final pressure by mass exchange through the interface between the liquid and the gas phases.
In this known method, the interface between the liquid and the gas phases is increased by injecting bubbles of the unwanted gas into the liquid to be degassed. When this known method is applied to water containing air, and when the bubbles are injected by known methods, the method has the drawback of requiring a length of time which becomes excessive when a large mass of water is to be degassed without excessively increasing the horizontal size of the degassing enclosure. This known method thus appears to be unacceptable on an industrial scale.
Preferred implementations of the present invention obtain high speed degassing at low cost in a degassing enclosure of small horizontal size and of simple construction.